Aircraft fuel system



Nov. 25 1947.

D. SAMIRAN 2,431,345

AIRCRAFT FUEL SYSTEM Filed Jan. 1, 1945 INVENTOR. 88 0, 1: (SIM/P4 Patented Nov. 25, 19 47 j UNITED "STATES PATENT OFFICE.v

pplicationzlgirlzy 1111435112!) No. 570,938

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370 O. G. 757) l The invention described herein may be manufactured and used by .or for the Government for governmental purposes, without the payment to me of any royalty thereon.

This invention relates to fuel systems having a plurality of fuel supply tanks and is particularly applicable to aircraft.

One of the advantages gained in a system of this kind by carrying the fuel supply in a number of separate tanks is that it makes for better weight distribution. Moreover, in combat craft there is the further advantage that if one tank becomes so punctured as to lose its fuel, the loss will be proportionately less as th number of tanks is greater.

There is, however, a slight disadvantage in a plural tank arrangement, in that when one tank becomes empty it frequently happens that, before a shift in connections may be made to another tank, a considerable volume of air has been drawn by the pump from the emptied tank and discharged into the fuel lines and carburetor. This disadvantage has given rise to the practice of supplementing the usual engine driven pump with an electrically driven auxiliary pump and with means which responds to a small drop in carburetor pressure to operate the electric pump until the engine driven pump may be reconnected to a full tank.

A system having an electric auxiliary pump and a fuel-pressure responsive electric switch for the pump is shown, described and claimed in my copending application Serial No. 475,683, filed February 12, 1943, now Patentv No. 2,406,854, September 3, 1946. In the system shown in this copending application, by Way of illustration, both the engine driven pump and the electric auxiliary pump are of the type which have an inbuilt relief valve for returning fuel from the discharge side back to the suction side upon a predetermined pump discharge pressure being exceeded, the relief valve of the engine pump being set, in the embodiment shown, at fifteen pounds, that of the electric auxiliary pump at ten pounds and the pressure responsive switch for operating the electric auxiliary pump being adjusted to start the electric pump when the discharge pressure from theengine pump falls below twelve pounds.

These relief valve pressures are, of course, arbitrarily selected for purposes of illustration. It is, however, desirable that the relief valve pressure ofthe engine pump be higher than that of the auxiliary pump and that the pressure at which the pressure responsiv switch becomes operative be between the two pump pressures.

In the operation of the system of the copendlng application, the engine pump takes fuel from a given tank until it is empty, then momentarily takes air from the tank and transfers it to the carburetor. The substitution of air for liquid in the discharge pipe and carburetor causes the pressure therein to drop considerably. The pressure responsive switch, also subject to carburetor pressure, senses the drop and starts the electric pump. The relief valve of the electric pump being set forten p. s. 1. pressure, cannot open the pressure responsive switch which is set at twelve p.. s. i. so it normally continues to maintain the fuel at the carburetor at ten p. s. i. until the engine pump may be reconnected to a full tank whereupon it picks up the fuel and builds up, in the dscharge pipe and carburetor, the fifteen p. s. i. to which it is set, whereupon the pressure responsive switch is actuated and the electric pump stops until the next emergency arises.

The foregoing operation of the system of the copending application will maintain only if the engine pump is immediately connected to a full tank. so that solid fuel at ten p. s. i. will be maintained at the carburetor until the engine pump regains full pumping capacity and operates the 4 pressure responsive switch to stop the electric pump. However, if there is any delay in selecting a full tank, as, for instance, when the selector cock is operated manually, the engine pump may, during the delay, build up an air pressure in the discharge pipe and carburetor which exceeds the pressure to which the electric pump is set, and thus override the electric pump and prevent its discharging any fuel into the system. This may result in a stalled engine,

It is therefore an object of this invention to provide an engine pump having air eliminating characteristics in the form of an inbuilt relief valve so constructed and arranged that it will not return any considerable liquid fuel from the discharge side back to the suction side until a relatively high pressure, say fifteen p. s. i., is exceeded but will return air or vapor from the discharge side back to the suction side when a pressure as low as several p. s. i. is exceeded.

Other objects and advantages will be evident as the invention is further described and reference is made to the drawings, wherein:

Fig. 1 is a schematic view of a system employing an engine driven pump to which my improved construction is adaptable.

Fig. 2 is a vertical axial section through a pump which embodies the principles of my invention.

Referring more particularly to the schematic view Fig. 1, an engine I6 carries a main fuel pump I2; the pump being operatively connected to the Outlet branch 26 is connected by a pipe 84 to,

the outlet opening of a main fuel tank 86, outlet branch 28 is connected by a pipe 88 to the outlet opening of an auxiliary fuel tank 46, and outlet branch 86 is connected by a pipe 42 to the outlet opening of a reserve fuel supply 44,

By providing a standpipe 46 on the upper end of the pipe 34, the fuel in the main tank 86 cannot be drawn below the line 48 by the pipe 84. The reserve supply 44 is therefore maintained in the main tank, although the reserve supply may be kept. equally well in a separate tank, if so desired.

The auxiliary pump 56 is driven by an electric motor 52 and'has its suction side connected to the bottom of the reserve supply 44, the discharge side being connected around the engine pump I2 and into its discharge pipe I6 by way of the pipe 54. A check valve 56 is biased for flow in i the direction of the arrow 58. The discharge side of the auxiliary pump 56 may also be connected by way of the pipe 42, selector cock 28, and pipe I8, to the suction side of the engine pump I2. 1.

Connected into the carburetor I4 by a pipe 56 is a gage 62 for visible indication of the pressure being pumped, and a pressure responsive switch 84 which comprises a body 66 divided into two compartments by a diaphragm 68 which is movable downward by the pressure in the pipe 66 but is urged upward by the spring 16, whereby, at asufllcient drop in pressure in the pipe 66, the spring moves the contact bar 12 into engagement with the contacts I4 and I6, thereby keeping the switch 64 closed as shown in Fig. 1 until the pressure in pipe 66 is restored. The spring III in theembodiment shown is so proportioned that the switch 64 closes upon a drop in pressure in the pipe 66 to about twelve pounds. A battery I8 is connectible to the contact I4 through a manual switch 86. The motor 52 and a signal light 82 receive current from the contact I6 when the switch 64 is closed; A small pipe 84 connects the air eliminating portion of the carburetor I4 to the top of the main fuel tank 86. The engine pump I2 and the mechanism which provides it with air eliminating characteristics will now be described.

Referring particularly to Fig. 2, the pump I2 comprises a body 86 with inlet opening 88 and discharge opening 86. The liner 82 has an ofiset bore. The rotor 84 is offset with the bore but concentric with its outside. The rotor 84 is radially slotted for the blades 96. A follower '88 keeps the outer ends of the blades against the bore of the liner.

The relief valve I66 comprises a valve head I62 which is spring loaded to keep it against a seat I64 in the body. When the pressure in the discharge space I66 exceeds the force of the loading springs, the valve head rises andpart or all of the fluid being pumped returns to the 76 The operation of the system shown in Fig.1

auction space I68. The usual bypass valve II6 consists of a disc II2 held by a spring H4 over a series of holes II6 and is provided for bypassing fuel from the suction side I68 to the discharge side I66 when the pump is inoperative.

The valve I66 is of the balanced type. A diaphragm' II 8 has its outer edge clamped between two parts of the body 86' by screws I26, its central portion being clamped between two discs I22. A cup-shaped guide member I24 is vertically slidable in a bore I 26 of the pump body 86. The two discs I22, the cup-shaped guide member I24, and a washer I28 are drawn together on the stem. I86 of the valve head I62 by a nut I82. The lower stem I84 of the valve head is guided inabore I86. 1

Within the cup-shaped guide member I24 is a smaller cup I88. Cup I88 is inverted and has an outwardly extending flange I46 surrounding the open end. A relatively strong coil spring I42 has its lower end resting on the flange I46 and itsupper end on the flange I44 of a nut I 46. A relatively weak coil spring I" has its upper end reacting against the closed end of the inverted cup I88 and its lower end against the washer I 28. A hollow screw I48 is threaded to flt the nut I46. Screw I48 has a head I56 by which it may be rotated to advance or withdraw the nut I46 and thereby adjust the spring I42. The strength of the larger spring I42 determines the pressure at which liquid fuel will be returned from the discharge side to the suction side of the pump. The strength of the smaller spring I" determines the pressure at which air or vapor will be returned from the discharge side to the suction side of the pump. A lock nut I52 clamps a flange I54 against a gasket I56 to seal against pressure leakage. The space within the bore I26 is usually connected to the carburetor intake whereby it is subjected to supercharger pressure. This connection is not shown since it forms no part of this invention.

A small adjusting screw I58, having a head I66 within the inverted cup I88, extends upwardly through the closed end of the cup and loosely through the hollow screw I48 and is provided with a screwdriver slot at its upp r end. Screw I58 is adjusted by a nut IN and secured in the desired position by a lock nut I62. Gaskets I54 seal against leakage past the screw I58.

The head I66 of the screw I58 acts as a stop to arrest expansion of the heavy spring I42 beyond a certain amount, the expansion being arrested just before it has completely closed the valve I66. The distance which the valve head I62 is still oif its seat I64 when the expansion of the heavy spring I42 is arrested by the head I66 depends, of course, on the adjustment of the nuts I6I and I62.

When the pump shown in Fig. 2 has been installed in the system shown in Fig. 1, the spring I42 is first adjusted by means of the hollow screw I48 until the valve passes fuel from the discharge a side back to the suction side when the discharge pressure exceeds fifteen pounds. The small screw I58 is next adjusted until the head I 66 lifts the inverted cup I38 until its flange I46 is raised from the washer I28 a distance of several one thouwhen equipped with a pump as shown in Fig. 2 will be substantially as follows:

Let it be assumed that the engine is at rest, the tanks are full and the fuel cock is turned so as to connect the auxiliary tank 40 through the pipes 38 and I8 to the suction side of the engine pump I2. If the system has been inactive long enough to allow the carburetor pressure to fall below twelve pounds, the automatic switch 64 will be closed as shown. The carburetor may now be provided with fuel at ten pounds pressure by closing the manual switch 86 and thus operating the electric auxiliary pump 56, which will deliver fuel to the carburetor through the pipe 54 and check valve 56. When the carburetor pressure reaches ten pounds, which may be observed by the guage 62, further operation of the electric pump will be of no avail since the relief valve in the electric pump will merely return the fuel pumped from the discharge side back to the suction side.

When the engine is now started, the engine pump I2 will supply fuel to the carburetor at fifteen pounds pressure which will open the automatic switch 64 and thus discontinue operation of the electric pump 56.

When the auxiliary tank 46 becomes empty, air will be drawn by the engine pump I2 through pipes 38 and I8 and discharged into the pipe I6. As soon as even a small volume of air is discharged into the pipe I6 the pressure will momentaril drop materially, sinc air is a compressible fluid, and this will cause the automatic switch 64 to close and the electric pump 56 to start and again deliver fuel to the carburetor I4 through the pipe 54 and check valve 56.

If the selector cock 24 is now immediately turned so as to connect the main tank 36 to the engine pump I2 through the pipes 34 and I8, the engine pump will pick up its prime and cause the small amount of air which has been delivered into the pipe I6 to be eliminated through the air eliminator pipe 84.

On the other hand, if the pump I2 was conventionally constructed without my improved relief valve I66 and there was any considerable delay in turning the relief cock 24 to the full tank 36, a fifteen pound air pressure would be built up by the pump I2, in the pipe I 6, the carburetor I4, and pipe 66, whereby the automatic valve 64 would stop the electrically driven pump and the engine would stop.

However, by providing an engine pump which is constructed as shown and described in Fig. 2 a, different result is had, for, when the auxiliary tank 46 becomes empty, the pump I2 will draw and discharge a small volume of air into pipe I6, the pressure in pipes I6 and 66 will momentarily drop, the automatic switch 64 will close, the electric pump will start and pump a ten pound pressure to the carburetor I4 through the pipe 54 and check valve 56, while in the engine pump I2, as soon as the air pressure in the discharge side I06 exceeds two or three pounds, the weak spring I41 will yield andv the valve head I62 will lift off its seat I04 until the washer I28 strikes the flange I40. The amount of lift is a matter of several one thousandths of an inch, as determined by adjustment of the small adjusting screw I56, and this slight opening of the valve I66 will return substantially all of the air being pumped by the engine pump I2 from the discharge side back to the suction side.

Engine operation may thus continue 011 the electric pump 56 until the pilot gets around to switching from the empty tank to a full one, whereupon the pump I2 will pick up its prime, deliver fuel to the carburetor at fifteen pounds pressure, and the automatic switch will open and stop the electric pump. Thereafter, in order that the relief valve I66 will pass the surplus liquid fuel from the discharge side back to the suction side, it will be necessary that it rise much higher than was required when pumping air only and this higher rise will be accomplished against the resistance of the stronger spring I42, which as before stated, was set to maintain a fifteen pound discharge pressure when liquid fuel is being pumped.

It will, of course, be understood that the pressures at which the several elements were adjusted was arbitrarily selected for illustrative purposes only, the essential condition being only that the engine pump maintain a higher pressure than the electric auxiliary pump and that the automatic switch willnot be opened by the pressure which the electric pump is capable of delivering, but will be opened by the pressure which the engine pump is capable of delivering.

Moreover, the small amount of rise of the valve head I62 against the resistance of the weak spring I41, before it must eflect further rise against the stronger spring I42, was given by way of illustration as several one thousandths of an inch, it may be more or less than this amount as the particular si uation requires, it being preferable, however, that when the pump is pumping air only, that the lift be at least enough to prevent the build up of air pressure at the discharge side of the engine pump which is equal to the pressure capacity of the electric pump.

Having shown and described an embodiment of my invention, I claim;

1. In a pump, a passageway for returning fluid from the discharge side back to the suction side, a valve in said passageway, a light spring arranged for moving said valve to the closed position but yieldable to a low pressure at the discharge side of the pump for letting said valve open slightly, a heavy spring expandi-ble for urging said valve toward the closed position, and a stop for arresting expansion of said heavy spring at a point just before said valve is completely closed by said heavy spring.

2. In a fluid pump, a passageway for returning surplus fluid from the discharge side back to the suction side, a poppet valve in said passageway biased to prevent flow through said passageway from the suction side to the discharge side, a relatively heavy spring expansible to move said poppet valve toward its seat, an adjustable stop for arresting expansion of said heavy spring when said valve is almost but not quite on its seat, and

' a light spring for moving said valve the remainder of its way onto its seat.

DAVID SAMIRAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PA'I'ENTS Number Name Date 1,519,856 Lorraine Dec. 16, 1924 1,753,024 Rode Apr. 1, 1930 2,126,874 Deschamps et al. Aug. 16, 1938 2,141,022 Rotter Dec. 20, 1938 2,196,500 Johnson Apr. 9, 1940 

